The present invention relates to a pressing method and a pressing device for forming a punched piece by punching a metal plate with a die and a punch.
Conventionally, as a pressing device related to this type of punching, there is a known device shown in FIGS. 6A to 6C, for example. This prior art pressing device includes a die 31 having a forming hole 311. A punch 32 is arranged to face the forming hole 311 to be able to reciprocate between the inside and the outside of the forming hole 311 in the die 31. As shown in FIGS. 6A to 6C, with a metal plate W1 placed between the die 31 and the punch 32, by punching operation of the punch 32, a punched piece W2 is formed by punching of the metal plate W1 by shear.
In this prior art pressing device, as shown in FIG. 6B, higher tensile stress acts on an inner peripheral portion of a punch area of the metal plate W1 pushed into the forming hole 311 than on other portions in forming a punched piece W2 by punching and especially at the initial stage of searing operation. In this way, as shown in FIG. 6C, when the punched piece W2 is formed by punching the metal plate W1, material at an edge of the punched piece W2 pushed into the forming hole 311 is moved by a large amount toward the outer side of the punch 32. As a result, the edge of the punched piece W2 is formed into an arcuate shape, i.e., shear droop W21 is formed. When the shear droop W21 is formed, a small protrusion, i.e., a burr W22 is formed at an edge portion of the punched piece W2 close to the punch 32. The burr W22 can be removed by grinding or the like after the punching. However, it is extremely difficult to correct the shear droop W21. If the punched piece W2 is used as a product, the function of the product may decrease due to the shear droop W21.
To cope with such a problem, there are conventionally proposed pressing methods or pressing devices as disclosed in Japanese Laid-Open Patent Publications No. 2000-117344 and No. 2000-210731, for example.
In the pressing method in Japanese Laid-Open Patent Publication No. 2000-117344, as shown with a long dashed double-short dashed line in FIG. 6A, the metal plate W1 is pressurized by a pressurizing tool 321, which has a larger outside shape than the inside shape of the forming hole 311 in the die 31, and the metal plate W1 is pushed into the forming hole 311 by a predetermined amount. In this manner, in the metal plate W1, a portion outward of the portion pushed into the forming hole 311 is reduced in thickness between the pressurizing tool 321 and the die 31. Then, as shown with the long dashed double-short dashed line in FIG. 6B, a punch 322, which has a smaller outside shape than the inside shape of the forming hole 311, shears the thin-walled portion to carry out punching.
Japanese Laid-Open Patent Publication No. 2000-117344 says that the shear droop W21 and the burr W22 are hardly formed by punching carried out in the above-described manner.
In the pressing device in Japanese Laid-Open Patent Publication No. 2000-210731, as shown in FIGS. 7A to 7C, a metal plate W1 is placed between a die 31 and a punch 32, and a punched piece W2 is formed by punching the metal plate W1. A protruding die 33 is arranged at the outer periphery of the punch 32. A hole 331, in which the punch 32 is housed to be able to reciprocate, is formed in a central portion of the protruding die 33. At a peripheral edge of the hole 331, a protrusion 332 is formed. The protruding die 33 can move in the same direction as the direction of movement of the punch 32.
Previous to forming of the punched piece W2 by punching with the die 31 and the punch 32, the protruding die 33 is moved toward the die 31 as shown in FIG. 7B. Then, by the protrusion 332 of the protruding die 33, an outer peripheral portion of a punch area of the metal plate W1 is sheared with a predetermined plate thickness remained uncut. Then, as shown in FIG. 7C, the punched piece W2 is formed by the punch 32.
In a pressing method in Japanese Laid-Open Publication No. 2000-210731, by shearing the metal plate W1 by the protrusion 332, material at an inner peripheral portion of the punch area of the metal plate W1 is restrained from moving toward the outer side of the punch 32. Japanese Laid-Open Patent Publication No. 2000-210731 says that the formation of shear droop W21 at the edge portion of the punched piece W2 is restrained by the shearing by the protrusion 332.
In Japanese Laid-Open Publication No. 2000-117344, the metal plate W1 is pressurized by the pressurizing tool 321, which has a larger outside shape than the inside shape of the forming hole 311, and is pushed into the forming hole 311. Therefore, even though the portion of the metal plate W1 outward of the portion pushed into the forming hole 311 is reduced in thickness between the pressurizing tool 321 and the die 31, the shear droop W21 is formed at the edge portion of the punched piece W2 in succeeding punching with the punch 32.
In the pressing method in Japanese Laid-Open Patent Publication No. 2000-210731, as shown in FIG. 7C, the metal plate W1 is pushed into the hole 311 by the punch 32. Therefore, although the metal plate W1 is sheared by the protrusion 332, it is impossible to restrain the formation of the shear droop W21 at the edge portion of the punched piece W2 in punching. In the pressing device in Japanese Laid-Open Patent Publication No. 2000-210731, the hole 311, in which the punch 32 is housed to be movable with respect to the protruding die 33, is formed in the central portion of the protruding die 33. Furthermore, the protrusion 332 for shearing of the punched piece W2 is formed around the hole 311. Therefore, in shearing the metal plate W1 by the protrusion 332, the protrusion 332 may become brittle and break.